1. Field of the Invention
This invention relates to a thermal printer using a thermal head of which a heating element is selectively driven.
2. Description of the Prior Art
Heretofore, thermal printers are constructed so that a thermal transfer recording may be performed on a material to be recorded by making use of a thermal head of which a heating element is selectively driven and by pressing the thermal head against an ink ribbon for thermal transfer use, or by pressing the thermal head directly against a sheet of thermal transfer recording paper.
The schematic construction of such a thermal printer as mentioned above is variously illustrated in FIGS. 1 through 3, respectively. In these drawings, 1 is a plate against which a sheet of recording paper 3 is pressed by paper-weight rollers 2; 4 is a cassette containing a thermal transfer ink ribbon 5; rectangular notch 6 (FIG. 3) is provided at the center of one side of cassette 4; ink ribbon 5 is exposed to the outside of the printer at notch 6; one end of ink ribbon 5 is fixed to take-up roller 7 and another end to feed roller 8, respectively; take-up roller 7 is rotated manually with knob 19 if occasion demands and at the same time it is rotated by ink ribbon take-up shaft 9 when printing forward (i.e., when printing toward the right hand side in FIGS. 1 and 3); 10 is a cassette holder which is fixed to carriage 11; cassette 4 is put into cassette holder 11 with the help of positioning pin 12; carriage 11 is guided to travel in the transverse direction as shwon in FIGS. 1 and 3, along guide shaft 13; carriage 11 is driven to travel as timing belt 16 suspended between driving pulley 14 and driven pulley 15 and fastened to carriage 11 is moved by motor 17 through driving pulley 14; driving pulley 14 and driven pulley 15 are provided with cogs in a desired shape and timing belt 16 is also provided at the inner surface thereof with cogs capable of gearing smoothly into the cogs of the two pulleys 14, 15; 18 is a thermal head arranged in the space between notch 6 of cassette 4 and ink ribbon 5 and the lower part of the thermal head is so pivotally attached as to be tilt-free to carriage 11; thermal head 18 is constantly biased by a spring (not shown) so as to keep clear of ink ribbon 5 (i.e., it is so biased as to keep such a position as shown in FIG. 2), and when printing forward (i.e., when printing toward the right hand side in FIGS. 1 and 3), thermal head 18 is forcibly tilted to the side of ink ribbon 5 by utilizing a solenoid or the like so as to press the heating element against the back surface (i.e., the uninked surface) of ink ribbon 5 and to press a sheet of recording paper 3 against platen 1 and thereby to bring the front surface of ink ribbon 5 into contact with the recording paper 3; 20 is a felt rod and 21 is a felt sheet, and they are brought into contact with both sides of ink ribbon 5 by utilizing the elasticity of leaf spring 22, so that dust and the like adhered to ink ribbon 5 may be removed thereby.
A recording operation with such a thermal transfer printer constructed as mentioned above is carried out by making carriage 11 travel in the transverse direction as shown in FIGS. 1 and 3 and intermittently driving platen 1 to rotate in the direction of the arrow shown in FIG. 1 every time when printing is completed on one transverse line. When printing backward, (i.e., when printing from the right to the left as shown in FIGS. 1 and 3), no thermal transfer is carried out, because thermal head 18 is separated from ink ribbon 5. With such a thermal printer as mentioned above, there are frequent occasions when high-density pica leters (hereinafter called HD pica) or high-speed pica letters (hereinafter called HS pica) are recorded in a HD pica mode or in a HS pica mode respectively set by an escape sequence applied from a host computer to which the thermal printer is connected. In the case of recording in the HS pica mode at a speed of the order of from 1.2 to 1.3 times faster than that in the HD pica mode, any of the density difference between the modes is negligible. However, when the speed is HS pica mode becomes 1.5 to 2 times or more faster than in HD pica mode, the recording density of the HS pica becomes lower than that of the HD pica, provided that a driving electric power applied to the heating element of the thermal head is kept constant. A similar problem also arises in the case of using such a thermal printer so constructed as a mode or to manually switch the HS pica mode over to the HD pica mode or vice versa. To cope therewith, for example, it is put into practice to shape up the pulse amplitude or pulse duration of driving electric power applied to the heating element of a thermal printer, every time by an operator, according respectively to each recording density of HS pica and HD pica.
According to such a conventional construction as mentioned above, however, a recording density adjusted by the operator does not correspond to a recording mode set by the host computer, if the recording mode varies during the recording operation, so that there may be a fear of recording density variations between the HS pica and the HD pica. In the case of switching a mode over to another by hand, a recording density is varied unless the density is adjusted every time by hand, thereby causing trouble.
This invention has been achieved by taking the above-mentioned points into consideration.